Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Dec. 31, 2023
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 856.9	$ 899.7		$ 899.7		$ 856.9
Collateral trusts, investments and cash	503.3	526.5		526.5		503.3
Surplus notes	340.0	340.0		340.0		340.0
Interest receivable	13.6	33.2		33.2		13.6
Debt	575.2	$ 564.7		564.7		575.2
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes				$ 503.0
Percentage of risk premium ceded		60.00%		60.00%
Renegotiation period				5 years
Percent over a target trust balance				102.00%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	288.6	$ 311.2		$ 311.2		288.6
Supplement trust target		603.0		603.0
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	568.3	588.5		588.5		568.3
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company
Guarantor Obligations [Line Items]
Guaranteed contracts, par value			$ 42.5		$ 42.5
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities						$ 150.0	$ 150.0	$ 100.0
Insurance-linked securities, term						12 years	12 years	12 years
Insurance-linked securities, callable term						7 years	5 years	5 years
Percentage of par value of policy reinsured						90.00%	90.00%	90.00%
Reinsurance, limit on total reimbursement						$ 150.0	$ 150.0	$ 100.0
Reinsured amount				$ 125.0
2018 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				24.00%
2021 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				29.00%
2022 Covered Portfolio | Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				30.00%
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Repayments of unsecured debt	36.0	0.0	$ 0.0	$ 0.0	$ 0.0
Accrued interest payment held in trust	1.0					1.0
Repayment of principal	24.6
Accrued interest payment held outside of trust	10.4					10.4
Debt	340.0	340.0		340.0		340.0
Interest receivable	$ 157.9	$ 177.6		$ 177.6		157.9
US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate				3.00%
Forecast | US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Interest rate									8.00%	7.70%
Maximum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured						276.7	301.7	276.1
Maximum | Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15.00%		15.00%
Minimum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation						$ 110.0	$ 135.0	$ 165.0